1. Field of the Invention
This invention relates to semiconductor transistor devices, and more particularly to submicron metal-oxide-semiconductor field effect transistors (MOSFETs), fabricated in ultra-thin silicon film.
2. Description of the Related Art
New techniques such as SIMOX (separation by implanted oxygen) layers have recently been developed for fabricating high quality thin silicon films on buried oxides. For example, see the article by the present inventors together with S. Seymour, "A High Performance Submicrometer CMOS/SOI Technology Using Ultrathin Silicon Films on SIMOX", 1988 VLSI Technology Symposium Tech. Digest. This technique has been used to improve radiation hardness and to promote volume inversion. An example of such a device is given in a published letter by Tsao et al., "Gate Coupling and Floating-Body Effects in Thin-Film SOI MOSFETs", Electronics Letters, Feb. 18, 1988, Vol. 24, No. 4, pages 238-39 ("SOI" refers to silicon-on-insulator). This letter discloses the fabrication of long channel (channel length of about 3 microns) n-channel MOSFETs with a thin silicon film 0.13 micron thick and channel doping concentration of 2.times.10.sup.16 /cm.sup.-3.
Typical ultrathin MOSFET designs with channel lengths below 1 micron require high dopant densities in the channel region to prevent device punchthrough. Punchthrough occurs when electric field lines from the drain extend toward the source and reduce the potential barrier height. This effect is normally prevented by sufficiently doping the channel so that the source is shielded from the drain by a space-charge region. For submicron devices the required dopant density substantially exceeds 10.sup.16 cm.sup.-3, and can be as large as 10.sup.17 cm.sup.-3. This level of channel doping has also been found to be useful in preventing short-channel effects, principally a threshold roll-off effect in which the device's voltage threshold falls off as the channel length is reduced. Another short channel effect overcome by doping the channel in this manner is a degradation in the sub-threshold current-voltage characteristic as the channel length is reduced, which is reflected by a reduction in turn-off performance. However, in addition to requiring the fabrication step of a channel implant, these devices are somewhat limited in terms of electron mobility and transconductance. Due to their subthreshold characteristics they also do not turn off as rapidly as might be desired.